Modelling key to optimising outcome of aluminium treatment

Aluminium treatment can be an efficient way to remove excess phosphorus from lakes and coastal waters. But if the external loading of phosphorus isn’t reduced, the effects of the treatment won’t last long, and monitoring and modelling are keys to optimising the results, concluded the researchers participating in last Baltic Breakfast.

Före och efter aluminiumbehandling av en sjö i Minnesota, USA. Foto: HAB Aquatics

Aluminium treatment has been used for nearly 60 years to restore eutrophic lakes. In total, likely thousands of treatments have been done – hundreds of them documented, describes Brian Huser, researcher at Swedish University of Agricultural Sciences (SLU), who has performed several studies on the topic.

In a lake that has a high external loading of phosphorus for a long time, the phosphorus accumulates in the sediment. If it is not bound to a mineral, it is released back to the water, and the sediments then becomes its own source for eutrophication in the lake.

Brian Huser, Swedish University of Agricultural Sciences

 – So, with the treatment, we basically restore the balance between minerals that bind phosphorus and phosphorus in the sediment. This keeps the phosphorus in the sediment and the internal phosphorus loading from the sediments returns to its natural level, explains Brian Huser.

The longevity of the aluminium treatment itself is infinite – the phosphorus stays bound to aluminium permanently, and is eventually buried with new sediment. But if external loading remains high, the new sediment will contain excess phosphorus and increase internal loading again. To get a long-lasting improvement on water quality, it’s therefore important that the external loading is reduced.


Benthic-feeding fish can affect longevity

To predict the longevity of a treatment on the levels of total phosphorus in the water, one can use the aluminium dose, the lake type (in a shallower lake the longevity typically is shorter, as it takes less internal loading to create eutrophic conditions) and the ratio between lake size and size of the watershed.

 – If you have a bigger watershed, you will have more external loading, and the bigger external loading – the quicker internal loading will come back, due to sedimentation of new excess phosphorus, says Brian Huser.

In a large study, Brian Huser and his colleagues has looked at the longevity of aluminium treatment in 114 lakes. In some cases, the predicted longevity differed significantly from the measured one, and these lakes all had in common that they had a medium to high biomass of benthic feeding fish, that dig in the sediment. 

 – They mix sediment and thereby increase the amount of phosphorus that can come out and cause problems with eutrophication.


Modelling is key for successful treatment

To ensure a safe and efficient aluminium treatment, Brian Huser emphasises, it is important with a proper design of the treatment. This can ensure that pH in the water doesn’t reach levels in which aluminium can have poisonous effects, but it can also save money. If the dose of aluminium added to the lake is unnecessarily high, a large portion of it is being crystalized before binding to phosphorus and thereby "wasted". In a poorly designed treatment, compared to a treatment with best possible design, ten times as much aluminium can be used to bind the same amount aluminium.

 – Good monitoring data and dynamic lake modelling are needed. We need to figure out which nutrient sources that are contributing to eutrophication and which ones need to be reduced. Otherwise, we are just guessing, adds Brian Huser.

Aluminium treatment in marine environment

The first full-scale aluminium treatment in marine environment was carried out by the Living Coast project (Levande kust) during 2012-2013 in the small and mostly shallow bay Björnöfjärden, in Stockholm archipelago. The treatment was found to be effective also in brackish water, and initially phosphorus concentrations in the water were significantly reduced and water quality improved.

The measures in Björnöfjärden have been accompanied by intensive monitoring, which has enabled researchers at Stockholm University Baltic Sea Centre to develop a physical-biogeochemichal model to further investigate the long-term effects of the treatment and the phosphorus dynamics in the bay, which is described in the recently published factsheet  Effekter på fosfor av aluminiumbehandlingen i Björnöfjärden (2103 Kb) .

The modelling results show that before the treatment there was about 400 kg phosphorus in total in the water. Annually, almost 900 kg of phosphorus settled to the sediments and almost all of it was released again back to the water.

 – This shows that phosphorus is used many times during the production cycle, says Bo Gustafsson, head of the modelling group Baltic Nest Institute.

Bo Gustafsson, Baltic Nest Institute, Stockholm University Baltic Sea Centre

During four years, during and after the treatment, more than 1700 kg of phosphorus has been bound to aluminium, and thereby ceased to circulate between the sediment and the water. The flux of phosphorus released to the water decreased to around 300 kg per year.


Changed dynamics of phosphorus

One of the main differences between a lake and a marine bay is that the bay has a continuous water exchange with the offshore. This means that when phosphorus levels in the bay decreases, for example as a result of an aluminium treatment, and the levels in the open sea remain high, there is a net import of phosphorus to the bay.

 – Some of the effect of the treatment is cancelled by a an additional "load" so to say from the offshore, says Bo Gustafsson.

Gradually the yearly amount of phosphorus that is bound to aluminium is decreasing, new organic matters is sedimented to the seafloor, releasing new phosphate to the water column, and the import of phosphorus remains high. 

 – The external loads are still too high, so the effect of the aluminium treatment is gradually going away.

The model enables comparisons between different scenarios, for example with or without treatment. This shows that the aluminium treatment improves the water quality in the bay for about a decade, but towards the end of the 2020s the difference between the scenarios is insignificant.


Reduction of land load is not enough

The model has also been used to investigate what would happen to the levels of total phosphorus in the bay if the land load of phosphorus was reduced with 50 percent, and if the phosphorus levels in the coastal waters outside of Björnöfjärden was reduced with 50 percent, respectively. The results indicate that neither one of these reductions alone is enough to keep the phosphorus levels below the boundary for Good-Moderate ecological status, as defined in the EU Water Framework 

 – It’s a challenging situation in these bays, says Bo Gustafsson.

 – The phosphorus concentrations in the open sea will decrease, our predictions say, but it will take decades. The local load reductions are in many places challenging and it might be significant delays there too, especially when it comes to phosphorus measures in agriculture. Aluminium treatment works, but as long as local loads and open sea concentrations are high, the effect will decrease with time.

Bo Gustafsson also emphasises that as the new model has been verified in Björnöfjärden, it can now be used to give an idea of the anticipated effect of aluminium treatment and other measures in other bays.

With good reduction of external loading, could we just wait and not treat with aluminium, asks moderator Gun Rudquist.

 – Well, it depends. How manageable are your loads and what are your goals? If you have an economically very valuable place maybe it is worth to do a treatment even if you only have an effect for 10-15 years. It’s a management issue, says Bo Gustafsson.

Brian Huser adds that in a city like Stockholm, it could be very difficult to install measures against the external loading, as it takes too much place. 

 – Then you can do these treatments and expect a longevity of 15 or 20 years and then come back and treat again.

Text: Lisa Bergqvist

Answers to questions from the audience

How do you respond when citizens are concerned about the toxicity of aluminium?

Brian: Aluminium is generally toxic when pH is lower than 5,5 or 6 or higher than about 9 or 9,5. There are other factors that can affect toxicity besides pH as well, most of these reduce toxicity potential, such as other elements in the water like organic matter. However, extreme pH conditions aren’t generally found in typical lakes. Eutrophic lakes can experience pH level above 9 due to high productivity of algae (they use CO2, which is an acid in water). However, after restoration there will be less algae, and pH will be substantially lower.

In addition, pH change in the sediment is buffered by chemical reduction processes that occur in the sediment in all lakes. Thus, even when pH is above 9 at the surface of the lake, it is closer to 7 in the sediment. This is the same for acidified lakes as well where we have seen large increases in sediment aluminium, even though the lake water has a pH of 4,5 or 5 (Huser and Rydin 2005). 

We can also look at what happens to aluminium in the water during and after a treatment. In Växjösjön, which was treated with aluminium in 2018, aluminium in the lake water decreased during, and especially after treatment. Heavy metals did as well. This is due to the fact that aluminium is naturally transported to lakes continuously, and will tend to "stick" to organic matter in the water. This includes algae. When we decrease algal growth during restoration, the amount of metals in the water decreases due to less organic matter in the water. 

How do you assess the toxicity of Al(OH)4 (pH >8)? Are there any publications on this?

Brian: There are of course many publications on aluminium toxicity, both at low and high pH. These are very easy to search for and find on the internet. Most of these are laboratory studies that do not take into account conditions in the lake or sediment. Before a treatment is conducted, a geochemical model should be run to assess any toxicity issues and minimize stress during treatment. Such modelling is also key to optimizing the efficiency of a treatment with respect to binding of phosphorus in the sediment. It is very important to make sure that any restoration measure is conducted as safely as possible. As stated above, once the aluminium mineral is at or in the sediment, pH is more neutral and closer to 7, even if pH is much higher (or lower) in the surface water. If pH is naturally above 9 or lower than 6, then a different method should be considered. 

You mentioned that several of the "failed" treatments involved lakes with a high proportion of carp that redistributed the sediment. Often eutrophic lakes have a high proportion of carp. How should this knowledge be managed? Under what conditions could this become a problem affecting the outcome of a mineral treatment (if the external load is choked)?

Brian: I would not use the word "failed" here, but that’s probably why it’s in quotes above. The aluminium still bound phosphorus permanently, and in fact we have shown that bottom feeding fish like carp are likely to improve binding efficiency of the aluminium mineral. This is due to the fact that carp mix the sediment, increasing the chance for the mineral to come into contact before it has substantially crystalized, thus increasing the amount of P bound per unit aluminium. The problem with carp is that they increase the mixing depth of the sediment. That is, the depth of sediment that release phosphorus to the lake water increases, and thus so does the amount of phosphorus that can be released from the sediment. Often we assume the uppermost 4 to 10 cm of sediment as the active layer that can release phosphorus, but carp can increase this to 15 cm (or maximum of 20+ cm, Huser et al. 2016). If this is not accounted for in dosing of aluminium, which it wasn’t for the studies where shorter longevity of treatment occurred, then the treatment will not last as long because there will not be enough aluminium mineral to bind all phosphorus that can be released by the sediment.

In the lake Lejondalssjön, Upplands-Bro, phosphorus treatment has been carried out on the bottom. Afterwards, a catastrophic explosion of increased spread and growth of the aquatic plant “axslinga” (Myriophyllum spicatum). From shores/reeds down to 4-5 m, virtually all around the lake, the bottom is almost completely covered by the plant. The consequence now is that it is not possible to swim, drive (electric) motors, fish or put crayfish cages along the shores around the lake. Even kayaking is dangerous, as there is a risk of getting your paddle stuck in the vegetation and tipping over. What happens when this vegetation dies and falls to the bottom or drifts towards the mudflats and beaches (oxygen consumption, eutrophication, suffocation of the bottom etc)? Has this been found in other places? What can be done about it? Does it depend on the method (surface/ground precipitation)? Is it a matter of stirring the bottom? Does it depend on the amount/number of traps treated? Are areas where the wind is on during treatment/afterwards worse affected. Who is responsible for negative impacts on residents, water owners, fishermen, etc?

Brian: This is something that is going to happen any time water clarity increases in a shallow lake (or shallow areas of a deeper lake), independent of what type of treatment method is used. It is also a classic debate between different groups that use lakes for different activities. Either we have macrophytes (aquatic plants) or algae in a lake. In unlucky cases sometimes both. Even if only external measures were needed, if lake water clarity increases, so will macrophtye growth. Aluminium treatment does not in itself promote macrophyte growth. The increase in water clarity does.

Expectations should be explained before a lake restoration project begins, however it is impossible to exactly predict how macrophytes will respond, but it is certain that their growth will increase. This is good for the ecosystem, provides shelter for fish and zooplankton that eat algae, reduces wind resuspension of sediment, and root systems help stabilize sediment even further. Macrophytes also provide a key food source for certain species of birds. But they can also hinder certain types of recreation lake uses.

With respect to wind, in shallow areas where wind increases the chance for resuspension of sediment, both sediment and the aluminium mineral will be resuspended and either deposited again in these shallower areas or transported to deeper areas of the lake. If shallow areas that are prone to sediment resuspension have excess sediment phosphorus, sediment injection of the mineral should be considered. In addition, as the aluminium mineral crystalizes, it becomes more stable and less likely to be resuspended. This is often why treatment is recommended later in the year before ice is formed in ice covered lakes. 

All dead organic matter will have a certain oxygen consumption as it is decomposed by bacteria, in natural or eutrophic systems. Often we see accelerated oxygen consumption in eutrophic lakes due to high production of algal matter and elevated accumulation of dead algae on/in the sediment. This is what causes release of phosphorus from iron. However low oxygen does not have an effect on phosphorus bound to aluminium. It is stable even if there is no oxygen in the water.

Was it correct that Bo's last illustration looked at aluminum seperate from land load and coastal load? If so, how would aluminum and land combined turn out?  

Bo: Yes, it is correct that the illustration shows measures separately. The effect from combined aluminium treatment and load reduction on winter TP would, at least on this time-scale, be approximately additive.

Is it not the case that the effect is infinate but it does not stop any future addition of phosphorus (if phosphorus is added, the effect from that isn’t handled)?

Bo:  Yes, the phosphorus bound to aluminium is removed from the system "forever", but phosphorus is flowing to and from the system all the time so therefore the effect from the treatment on phosphorus concentrations will diminish with time.

See a recording of the webinar here